This invention relates to performance testing of transmission systems, and more particularly, to the performance testing of CATV (Community Antenna TeleVision) systems.
In order to ensure quality viewing by CATV subscribers, frequent testing must be undertaken of television processing and transmission equipment. Traditionally the testing has included quantification of transmission impairments, such as carrier to noise ratio, amplitudes of distortion products and spurious signals, and the overall and channel frequency response of the system.
To prepare for prior testing, it has been necessary to rewire the transmission headend in order to properly introduce required test signals. Not only is this time consuming, but it also is expensive and a source of interference with the system users.
Measurements of test parameters often are made with a spectrum analyzer both at the headend and in the field. In order to measure television noise and underlying distortion products, the channel being tested is turned off so that the test parameters may be determined without interference.
This results in interruption of subscriber viewing and requires manual removal and restoration (turning off and on) of the selected channel at the headend.
Typically there are a number of test channels (4 to 20) that need to be addressed and several (6 or more) testing locations.
These operations are repeated many times, necessitating full attention at the headend for the duration of testing, generally a full day or more. In addition, communications from the field to the headend are required in order to synchronize switching and other operations. This usually is done by radio or telephone and results in delays from over crowded communication channels.
These procedures are laborious and labor intensive, often generating customer dissatisfaction because of frequent viewing interruptions.
Accordingly, it is an object of the invention to facilitate transmission testing. A related object is to facilitate transmission testing in community antenna television (CATV) systems.
Another object of the invention is to reduce the labor required in transmission testing, as well as achieve financial savings and the elimination of viewing interruptions.
Still a nother object of the invention is to promote customer satisfaction, particularly for television transmission.
A further object of the invention to provide for measurements of cable television system operating parameters with reduced manpower.
Yet another object is to allow testing without personnel at the headend and without the need for communications to and from the tester at the test points throughout a CATV system.
Still another object is to insert and use test signals without interference to the TV pictures viewed by subscribers.
A further object is to permit measurements of actual system noise and other underlying interference without interruption of subscriber viewing.
A still further object is to obviate the need for setup time and labor each time the system is to be tested.
Another object is to provide equipment, which in the event of equipment power supply failure, will not disrupt the functioning of a CATV system.
In accomplishing the foregoing and related objects, the invention provides a method and system for transmission testing by a permanent installation for the introduction of test signals. Required manpower for testing performance is greatly reduced and interference with customer viewing is virtually eliminated.
The apparatus of the invention includes a control unit installed in the headend and wired to the processing equipment of each test channel. This control unit includes equipment for the generation of required video test signals, a processor to automatically sequence the insertion of test signals and generate additional signals which cause certain other units to function properly. The panel of the control unit also has a display which assists in the setup of sequences and indicates current operating status.
In addition to the control unit, a switching unit is used in conjunction with the processing equipment employed on each test channel. These units work in concert with video insertion sequencing to turn off the radio frequency output of the channel for a short period while the measurement of the underlying signals is performed.
Turning off the test channel visual (or visual and aural) radio frequency carrier(s) allows measurement of underlying undesirable and interfering signals that interfere with subscriber viewing. From this perspective, the carrier desirably is removed only during non-informational or viewable time intervals, which, for television, is known as the xe2x80x9cvertical intervalxe2x80x9d (usually lines 10-22 of both television fields). The xe2x80x9coffxe2x80x9d period produced by this switching is generally less than one television line, although modern TV receivers will tolerate xe2x80x9coffxe2x80x9d periods which remove several horizontal synchronizing pulses without any noticeable picture degradation. However, longer xe2x80x9coffxe2x80x9d periods can begin to be noticed as interruptions in the aural program.
Since the technique of the invention is employed to measure carrier to noise ratio (C/N), composite triple beat CTB), as well as composite second order (CSO) and spurious signal amplitudes, these impairments are collectively designated simply as xe2x80x9cCTBxe2x80x9d, which are uniquely measured by sampling.
In order to complete the measurement process of the invention, a demodulator/decoder is used in conjunction with a spectrum analyzer (or other detection equipment) that executes measurements. The demodulator/decoder activates a switch in series with the measuring equipment only during the xe2x80x9coffxe2x80x9d periods of the test channel carrier to allow measurement of signals without test channel regular signal intrusion.
The invention provides for measurements of cable television system operating parameters with expenditure of reduced manpower by providing automatic sequencing of test signals and conditions to allow testing without personnel at the headend and without the need for communications to and from the tester at the test points throughout the CATV system.
Test signals are inserted and used within the television vertical interval to prevent interference with the TV pictures viewed by the subscribers, and measurements are made of actual system noise and other underlying interference without interruption of subscriber viewing.
The invention also develops a permanent headend installation that obviates the need for setup time and labor each time the system is to be tested, and headend equipment is provided which will not disrupt the functioning of the CATV system in the case of equipment power supply failure.